An adsorbable spectral sensitizing dye is generally used for spectral sensitization of silver halides, and the spectral sensitization of silver halides can be attained by the introduction of photoexcited electrons thereinto from the dye adsorbed on the surface of the silver halide.
As spectral sensitizing dyes there are widely used methine series dyes to which has been imparted adsorbability and which have an appropriate redox potential, for example, cyanines, merocyanines, complex cyanines and complex merocyanines. However, spectral sensitization with such adsorbing dyes is limited with respect to the spectral sensitization degree because of the limited amount of the sensitizing dye which is capable of being adsorbed on the surface of silver halides, and, further, it is known that the saturated adsorption or nearly saturated adsorption of the dye often causes extreme desensitization (color desensitization).
In view of the above, a method of spectral sensitization with non-adsorbing dye molecules in which adsorption of the dye on the surface of silver halides is not required but energy transference from a non-adsorbed dye molecule to an adsorbed sensitizing dye molecule is utilized for attaining spectral sensitization with a non-adsorbed dye molecule was developed. See, for example, Japanese Patent Application Nos. (OPI) 117619/76 and 239143/87 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") and Japanese Patent Application Nos. 284271/86 and 284272/86.
In accordance with such a method, silver halide grains are previously spectrally sensitized to optimum sensitivity with an adsorbed spectral sensitizing dye, and then an energy transferring type dye is added to the binder used in high concentration so that the desired increase in spectral sensitivity is attained utilizing the light-collecting effect of the energy transferring dye. Sensitization of this type is hereafter referred to as "light-collecting sensitization".
In light-collecting sensitization, a remarkable light-collecting effect can be attained in a system where the concentration of the energy transferring type dye (hereafter referred to as a "light-collecting dye" or "LC dye") in the emulsion binder is sufficiently high. In addition, regarding the adsorbing sensitizing dye which is an acceptor of energy, the use of tabular grains which have a large relative surface area and which can adsorb a large amount of dye in a high amount is effective for more efficiently attaining light-collecting sensitization. In other words, the emulsion system where a larger amount of the spectral sensitizing dye is adsorbed onto the emulsion grains is more effective for attaining sufficient light-collecting sensitization.
However, in accordance with the conventional light-collecting sensitization process, since the adsorbing spectral sensitizing dye which is an energy acceptor on silver halide is added to the emulsion in the last stage after the completion of the formation of the light-sensitive silver halide in a conventional manner for effecting spectral sensitization, adsorption of the dye to the surface of the silver halide is weak and, as a result, there are various problems which interfere with the light-collecting sensitization.
One of these problems is that a part of the spectral sensitization dye often remains free in the binder in the non-adsorbed state because of the weak adsorption power thereof and the free dye acts as a quenching agent to the LC dye in the binder so that the energy transfer to the adsorbed dye is prevented and the light-collecting sensitizing effect is lowered.
Another problem is that release of the spectral sensitizing dye often occurs when the amount of the dye generally exceeds about 50% of the saturated amount of the dye capable of being coated on the surface of the silver halide. As a consequence, the effective amount of the adsorbing spectral sensitizing dye as an energy acceptor is limited. This means a reduction of the amount of the sensitizing dye adsorbed onto the silver halide grains and, as a result, the light-collecting sensitizing effect is naturally limited.
Under these circumstances, the present inventors found that a silver halide emulsion can be more effectively sensitized by a light-collecting spectral sensitization method as described hereafter, in which the adsorption force of a spectral sensitizing dye, as an energy acceptor, onto silver halide grains is increased and the silver halide grains are sufficiently spectrally sensitized with the dye and are processed for light-collecting sensitization.